Photosynthetically active radiation is any radiation that can be used for photosynthesis. This is generally wavelengths in the visible range (400-700nm). Radiation outside this range is given zero weight. This is slightly different than photometry, which has fractional weights depending on the wavelength, not just binary weights (0 or 1).
Introducing our new Applied Spectroscopy Division
ChemWiz-ADK
Handheld NIR Spectrophotometer with on board chemometrics and spectral matching!